This invention relates to the measurement of electromagnetic signals originating in the human body, and, more particularly, to the measurement of magnetic and/or electrical fields originating from brain activity.
The human body produces various kinds of energy that may be used to monitor the status and health of the body. Perhaps the best known of these types of energy is heat. Most healthy persons have a body temperature of about 98.6.degree. F. A measured body temperature that is significantly higher usually indicates the presence of an infection or other deviation from normal good health. A simple medical instrument, the clinical thermometer, has long been available to measure body temperature.
Over 100 years ago, medical researchers learned that the body also produces electrical signals. Doctors today can recognize certain patterns of electrical signals that are indicative of good health, and other patterns that indicate disease or abnormality. The best known types of electrical signals are those from the heart and from the brain, and instruments have been developed that measure such signals. The electrocardiograph measures electrical signals associated with the heart, and the electroencephalograph measures the electrical signals associated with the brain. Such instruments have now become relatively common, and most hospitals have facilities wherein the electrical signals from the bodies of patients can be measured to determine certain types of possible disease or abnormality.
More recently, medical researchers have discovered that the body produces magnetic fields of a type completely different than the other types of energy emitted from the body. The research on correlating magnetic fields with various states of health, disease and abnormality is underway, but sufficient information is available to demonstrate that certain emitted magnetic fields are associated with conditions such as epilepsy. Present medical studies are investigating the nature of the normal and abnormal magnetic fields of the brain, and seeking to correlate those fields with brain functions and patient health.
For example, if it were known that a particular condition, such as epilepsy, were associated with an abnormal magnetic field produced at a particular location in the brain, then it might be possible to detect the abnormality at an early stage, before the condition became acute, and then apply other medical knowledge to treat or surgically remove that precise portion of the brain with minimal side effects on the patient. A selective measurement of brain activity could also permit more precise use of drugs to control the condition. Magnetic studies of the brain therefore offer the potential for understanding and treating some of the most crippling diseases and conditions known.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain and heart. The biomagnetometer is a larger, more complex instrument than the medical instruments mentioned earlier, primarily because the magnetic fields produced by the body are very small and difficult to measure. Typically, at 1 centimeter from the head, the strength of the magnetic field produced by the brain is about 0.000000001 Gauss. By comparison, the strength of the Earth's magnetic field is about 0.5 Gauss, or five hundred million times larger than the strength of the magnetic field of the brain. Most electrical equipment also produces magnetic fields, in many cases much larger than that of the Earth's field. It is apparent that, unless special precautions are taken, it is not possible to make magnetic measurements of the human body because the external influences such as the Earth's magnetism and nearby apparatus can completely overwhelm and mask the magnetic fields from the body.
The biomagnetometer includes a very sensitive sensor for magnetic signals. The currently most widely used sensor is a Superconducting QUantum Interference Device or SQUID, which is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) This detector and its associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body.
The present biomagnetometer therefore provides a chair or table for the patient, and a structure which places the detector in proximity with the head of the patient, as about 1-2 centimeters away. Special electronics is provided to filter out external effects such as the Earth's magnetic field and the magnetic fields of nearby electrical instruments. (For a description of such a device, see U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference.) The patient and detector can also be placed into a magnetically quiet enclosure that shields the patient and the detector from the external magnetic fields. (For a description of such an enclosure, see U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference.) With these special precautions, medical researchers and doctors can now make accurate, reliable measurements of the magnetic fields produced by the brain, and are studying the relationship of these fields with diseases and abnormalities.
It is well established that certain physically identifiable locations in the brain are responsible for specific types of activities and functions. It is therefore important to correlate the measured biomagnetic field with the particular location in the brain which produces the field. Such a correlation is important to understanding the mechanism by which disease and disorder arise, and also to the treatment of the problem.
Correlating the spontaneous measurement taken by an external array of magnetic or electrical sensors with brain activity at a specific location within the brain is difficult, primarily because other areas of the brain continue to function and produce their own magnetic and electrical fields, even as a measurement is being taken with the intent of measuring activity at a specified location, and because the measurement sensors and instrumentation produce noise that may be of the same magnitude as the signals to be measured. It is not easily determined whether a particular signal measured externally originates at the selected location, other locations, or jointly at the selected location and other locations, or in fact is a manifestation of instrument noise. At the present time, there is a good deal of reliance on averaging multiple occurrences of magnetic and/or electric signals synchronously with external events such as a stimulus, to isolate the origin of particular magnetic signals.
Moreover, it is difficult to develop data from spontaneous brain activity, having a high signal to noise ratio, that can reliably be said to originate at a selected location in the brain. Having such a capability would be extremely useful, because it would permit studies of neurological disorders associated with epilepsy, stroke, and head injury, for example, and even direct physiological studies of some of the most basic phenomena of life, such as attention and boredom, mental disorders, language comprehension and expression, and response to external stimuli.
An important step in correlating external measurements with the specific locations of internal events is disclosed in U.S. Pat. No. 4,793,355, whose disclosure is incorporated by reference, and which provides a methodology for automatically tracking the position of the sensors in respect to the position of the head of the patient. When used in conjunction with the known spatial sensitivity profile of the detector and either an external stimulus or synchronization with voluntary activity, this approach gives important information about the internal origin of the externally measured signal. This technology, by itself, is limited in its resolution of location and nature of the source, because of various types of noise and the continued operation of other brain functions as measurements are taken. It is also limited in its ability to investigate spontaneous, non-evoked brain activity.
There is therefore a need for an improved approach for measuring biomagnetic fields and correlating those fields with their source location within the brain. Preferably, such an approach would permit data to be simultaneously obtained for a number of different signals produced from sensors in a number of different locations, to permit correlation of all signal information. The approach also must achieve a high signal to noise ratio that permits the signal of interest to be discerned and isolated relative to other brain signals, external noise, and instrument noise. The present invention fulfills this need, and further provides related advantages.